A variety of types of impellers have in the past been used for pumps for use in pumping liquids having a high entrained solids content, typical of these being twin vaned centrifugal pumps and vortex pumps. The former are somewhat prone to "ragging" i.e. to entrapment of the solid content on or within the impeller whereas the latter, while significantly less prone to ragging, operate inefficiently. Single vane impellers operating on a substantially axial feed are known as for example in U.S. Pat. No. 3,156,190 which describes a spiral screw blade encircling a surface of a flared hub or core portion, but while such impellers can be constructed to function to good effect and efficiency in pumping liquids having a high solids content without significant ragging, the performance of such impellers can in general not be adjusted without considerable difficulty and expense and even then rarely without operation of the pump set at reduced efficiency.
The technique of cutting a centrifugal impeller to modify its pumping performance, for example its pressure head generated at a given rotational velocity, is well known although for reasons largely concerned with impeller balance is primarily of interest in the modification of multivaned, inherently balanced impellers. The cutting of substantially axial impellers is rarely feasible or effective.
The prior art discloses a number of impellers for a variety of different purposes having spiral or helical type vanes encircling a flared hub or core portion, the impellers being for a variety of different purposes. For example U.K. Patent Specification No. 174184 discloses a multivaned substantially centrifugal impeller of the closed or shrouded type for water, U.S. Pat. No. 3,035,781 discloses an impeller for a pulper for suspending dried pulp in water, U.S. Pat. No. 3,644,056 discloses a centrifugal pump having a vaned impeller for unspecified liquids and U.K. Patent Specification No. 1153993 discloses a rotary impeller pump having a high speed impeller intended as a super-cavitating pump for aircraft fuel systems. None of these specifications disclose, appreciate or suggest the problem facing the present applicants, namely to produce a single vane impeller for pumping liquids having a high entrained solids content, the impeller being adjustable by cutting to modify its pumping performance without detriment to its solids handling ability and without significant detriment to its pumping efficiency. The requirement for the impeller to be single vaned itself arises from the need for the impeller to be non-ragging, the use of a plurality of vanes of necessity introducing points of interception of surfaces or edges as locations where ragging can occur, and also restricts the size of solid that can effectively be pumped.